The present invention relates to a rotating anode and more particularly to means for flowing out the anode current of the tube, in particular when the tube is of the magnetic bearing type.
The X-ray tubes for medical diagnosis, for example, are generally constituted as a diode, ie. with a cathode and an anode or an anti-cathode, electrodes being enclosed within a vacuum sealed casing which ensures electrical insulation between these two electrodes. The cathode produces an electron beam and the anode receives such electrons on a small surface forming a focal point, the X- rays are emitted therefrom.
When the power supply high voltage is applied across the terminals of the cathode and the anode, so that the cathode be at a negative potential, a so-called anodic current is established in the circuit, through a generator that generates the power supply high voltage; the anodic current passes through the space between the cathode and the anode in the form of an electron beam which impinges onto the focal point.
A small proportion of the energy used for generating the electron beam is converted into X-rays, the remainder of the energy being dissipated into heat. Thus, also taking account of the high levels of instantaneous power involved (in order of 100 KW) and the small size of the focal point (in the order of 1 mm), manufacturers have for a long time produced rotating anode X-ray tubes wherein the anode is put into rotation for the thermal flow to be distributed over a crown or ring referred to as the focal crown or ring the area thereof being far larger than the focal point; the advantage becomes all the greater as the speed of rotation rises (in general between 3,000 and 12,000 revolutions per minute).
The conventional rotating anode is in the general form of a disc having an axis of symmetry about which it is put into rotation by means of an electric motor; the electric motor has a stator located outside the casing and a rotor mounted within the casing of the X-ray tube arranged along the axis of symmetry with the rotor made mechanically integral with the anode through a supporting shaft.
According to another well known construction still of very common use, the rotor is mounted on mechanical bearings provided with ball bearings. It is known that X-ray tubes of the mechanical bearing type have a shortened life span due notably to the wear of the ball bearings; one of the causes of wear is lubrication, which cannot be accomplished perfectly notably because of the vacuum existing within the X-ray tubes. However, one of the advantages of the mechanical bearings provided with metal ball bearings lies that there is conductive material contact between the rotating parts (rotor, anode) and the fixed parts of the tube (rotor support shaft, casing); this material contact is achieved by the bearing balls and at the same time constitues electrical contact so that the anodic current of the X-ray tube can flow out.
To overcome the problems raised by from the fast wear of mechanical bearings, a major improvement consists in assembling the rotating, more specifically the rotor, with magnetic bearings. Generally, these include electromagnets mounted in pairs opposited by which generate magnetic fields under the influence of which the rotor, integral with the rotating anode which rotated thereby, is maintained in a state of balance; the rotating anode and the mechanical parts in rotation therefore have no further material contact with the remainder of the X-ray tube.
The advantages of magnetic bearings when applied to the rotation of anodes are mainly the absence of noise, the absence of vibration and the possibility of obtaining a significantly increased lifetime of the rotating system.
But with magnetic bearings, the rotating anode is mechanically and electrically isolated from the fixed parts of the X-ray tube means so that implementation of means specifically intended for outflow of the tube anodic current is required.
For this purpose, it is known to use the emission of electrons produced by one or several auxiliary thermo-emissive cathodes, linked mechanically with the rotating anode; these electrons are captured by one or several auxiliary anodes in fixed positions. One of the main difficulties is then to supply these auxiliary cathodes, put into rotation, with the energy needed to raise their temperature up to a sufficiently high level so as to satisfy the laws of thermo-electronic emission. In addition, solutions of this type are comparatively complex and expensive.
It is also known to use friction systems whereby electrical contact between the rotating parts and fixed parts of the X-ray tube is obtained by the friction of two parts against one another, one coupled in rotation with the rotating anode and electrically connected to the latter, and the other being a part in a fixed position electrically connected to the positive polarity of the X-ray tube power supply high voltage. But the latter solution, although it represents a rugged solution easy to build, is nevertheless a mechanical friction solution which implies its wear to grow much faster than the wear of magnetic bearings thus also tending to shorten the possible lifetime of the X-ray tube.